In recent years, it has become common to transmit large-volume data such as still image data and moving image data in addition to audio data in cellular mobile communication systems, in response to the spread of multimedia information. In LTE-Advanced (Long Term Evolution Advanced), active studies have been conducted to achieve a high transmission rate using wideband radio band, MIMO (Multiple-Input Multiple-Output) transmission techniques and interference control techniques.
After completion of the acquisition of the parameter specific to the base station (which may also be referred to as “eNB”), the terminal (which may also be referred to as “UE: User Equipment”) sends a connection request to the base station and thereby establishes communication with the base station. The base station transmits control information to the terminal with which communication has been established via a downlink control channel such as PDCCH (Physical Downlink Control Channel) as appropriate.
The terminal then performs “blind detection” of a plurality of pieces of control information (which may also be called “downlink control information (DCI)”) included in the received PDCCH signal. That is, the control information includes a CRC (Cyclic Redundancy Check) portion and this CRC portion is masked with a terminal ID of the transmission target terminal by the base station. Therefore, the terminal cannot determine whether or not the received control information is control information intended for the terminal until the terminal demasks the CRC portion with the terminal ID of the terminal itself. When the demasking result shows that CRC calculation is OK, it is determined in this blind detection that the control information is intended for the terminal itself. The downlink control information includes DL (downlink) assignment indicating assignment information of downlink data and UL (uplink) grant indicating assignment information of uplink data, for example.
Next, an uplink retransmission control method in 3GPP LTE will be described. In LTE, there is an FDD (Frequency Division Duplex) system and a TDD (Time Division Duplex) system. In the FDD system, a downlink component carrier (downlink CC) and an uplink component carrier (uplink CC) are allocated to different frequency bands.
In the TDD system, a downlink component carrier and an uplink component carrier are in the same frequency band, and the TDD system realizes downlink communication and uplink communication by switching between downlink and uplink in a time-division manner. For this reason, in the TDD system, a downlink component carrier can also be expressed as “downlink communication timing in a component carrier.” An uplink component carrier can also be expressed as “uplink communication timing in a component carrier.” Switching between the downlink component carrier and the uplink component carrier is performed based on a UL-DL configuration as shown in FIG. 1
The UL-DL configuration is indicated to the terminal by a broadcast signal called “SIB1 (System Information Block Type 1)” (SIB1 indication), the value thereof is the same throughout the entire system and the value is not expected to be changed frequently. In the UL-DL configuration shown in FIG. 1, timings in units of subframes (that is, units of 1 msec) are configured for downlink communication (DL: Downlink) and uplink communication (UL: Uplink) per frame (10 msec). The UL-DL configuration allows for building a communication system that can flexibly respond to requests for throughput for downlink communication and throughput for uplink communication by changing a subframe ratio between downlink communication and uplink communication. For example, FIG. 1 illustrates UL-DL configurations (Config#0 to 6) with different subframe ratios between downlink communication and uplink communication. In FIG. 1, a downlink communication subframe (DL subframe) is represented by “D,” an uplink communication subframe (UL subframe) is represented by “U” and a special subframe is represented by “S.” Here, the special subframe is a subframe when a downlink communication subframe is switched to an uplink communication subframe. In the special subframe, downlink data communication may also be performed as in the case of a downlink communication subframe.
In LTE, a retransmission control method called “synchronous HARQ” is used in uplink to reduce the number of bits in a control signal. With synchronous HARQ, UL subframes whose UL HARQ processes are identical are determined in advance and when carrying out uplink retransmission, retransmission is carried out in UL subframes corresponding to the identical UL HARQ process. By so doing, a base station can select data to be retransmitted from a terminal without explicitly indicating, to the terminal, which of data transmitted in the past should be retransmitted. However, synchronous HARQ has a mechanism whereby uplink data transmitted in the past can be retransmitted only in a UL subframe of an identical process.
In LTE, the FDD system and TDD system assign different process numbers to UL HARQ processes, respectively. The TDD system predefines process numbers which differ depending on TDD UL-DL configurations (e.g., FIG. 1). In FIG. 1, a number assigned below a UL subframe (“U”) indicates a process number of a UL HARQ process associated with the UL subframe. For example, in Config#0, the number of UL HARQ processes is 7 and UL HARQ processes of process numbers #1 to #7 (which may also be expressed as “UL HARQ processes #1 to #7” hereinafter) are assigned to UL subframes in order. In Config#2, the number of UL HARQ processes is 2 and UL HARQ processes #1 and #2 are assigned to UL subframes in order. The same applies to Config#1 and Config#3 to Config#6. In all UL HARQ processes, these numbers of UL HARQ processes are set to a minimum number of processes when an interval after uplink data is transmitted in a UL subframe until retransmission is indicated in a DL subframe is the fourth or after the fourth subframe, and at the same time an interval after retransmission is indicated in a DL subframe until retransmission data is transmitted in a UL subframe is the fourth or after the fourth subframe. Thus, a UL-DL configuration which includes many UL subframes has more UL HARQ processes and a UL-DL configuration which includes fewer UL subframes has fewer UL HARQ processes.
FIG. 2 illustrates the number of UL HARQ processes of each UL-DL configuration (Config#0 to Config#6) and a cycle (UL HARQ cycle, time [ms] or the number of frames) indicating an interval at which an association between a subframe number and a process number corresponding to the subframe number becomes identical.
In the LTE-Advanced system, studies are being carried out on changing UL-DL configuration (hereinafter referred to as “TDD eIMTA (enhancement for DL-UL Interference Management and Traffic Adaptation),” which may also be referred to as “dynamic TDD” or “flexible TDD”) (e.g., see NPL 1). Exemplary purposes of TDD eIMTA include provision of a service that meets the needs of users by flexible changes of a UL/DL ratio or reduction in power consumption at a base station by increasing the UL ratio in a time zone when traffic load is low. As a method of changing UL-DL configuration, the following methods are under study in accordance with the purpose of change: (1) method using indication of an SI (System Information) signaling base, (2) method using indication of an RRC (higher layer) signaling base, (3) method using indication of a MAC (Media Access Control layer) signaling base and (4) method using indication of an L1 (Physical Layer) signaling base.
Method (1) is to change the least frequent UL-DL configuration. Method (1) is suitable for a case where the purpose is to reduce the power consumption at a base station by increasing the UL ratio, for example, in a time zone when traffic load is low (e.g., midnight or early morning). Method (4) is to change the most frequent UL-DL configuration. The number of terminals connected is smaller in a small cell such as a pico cell than in a large cell such as a macro cell. In a pico cell, UL/DL traffic in the entire pico cell is determined depending on the level of UL/DL traffic in a small number of terminals connected to the pico cell. For this reason, UL/DL traffic in the pico cell fluctuates drastically with time. Thus, method (4) is suitable for a case where UL-DL configuration is changed to follow a time fluctuation of UL/DL traffic in a small cell such as a pico cell. Method (2) and method (3) are positioned between method (1) and method (4) and suitable for a case where UL-DL configuration is changed with medium frequency.